1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same in which isolation is performed by a field oxide.
2. Description of the Prior Art
Although isolation methods are roughly classified into PN junction isolation and dielectric isolation, the dielectric isolation has been popularly used compared with the PN junction isolation. In the dielectric isolation, oxide isolation performed by a field oxide formed by local oxidation of silicon (LOCOS) method is most popularly used.
FIGS. 1A and 1B show the steps in manufacturing a conventional stacked capacitor DRAM in which isolation is performed by a field oxide formed by the above LOCOS method. In order to manufacture the conventional DRAM, as shown in FIG. 1A, an SiO.sub.2 film 12 serving as a field oxide is formed on the surface of the isolation region of an Si substrate 11 by the conventional known process of the LOCOS method first, and an SiO.sub.2 film 13 serving as a gate oxide is formed on the surface of an active region.
As shown in FIG. 1B, a gate electrode (i.e., a word line) of a MOS transistor 14 constituting a memory cell is formed by a polysilicon film 15 on the SiO.sub.2 films 12 and 13. Thereafter, N.sup.- -type layers 16 and 17 each serving as the source/drain of the MOS transistor 14 are formed in a P-type well of the Si substrate 11 while the polysilicon film 15 and the SiO.sub.2 film 12 are used as masks.
An insulating film 21 is deposited on the entire surface of the resultant structure, and the entire surface of the insulating film 21 is etched back to form side walls constituted by the insulating films 21 on the side portions of the polysilicon films 15. Thereafter, a capacitor constituting a memory cell, bit line, and the like are formed to complete the conventional stacked capacitor DRAM. Note that, after the SiO.sub.2 film 12 is formed by the LOCOS method, the entire surface of the SiO.sub.2 film 12 may be etched without using a mask.
When the SiO.sub.2 film 12 is formed by the LOCOS method, however, it is known that a bird's beak 12a is formed at an end of the SiO.sub.2 film 12. In addition, as shown in FIG. 1A, the Si substrate 11 under the bird's beak 12a has a latent stress 22 or microdefects such as heavy metal contamination and cleavage of the bonds between atoms.
For this reason, when the Si substrate 11 is damaged by etchback for forming the side walls constituted by the insulating films 21, crystal defects 23 such as dislocation and stacking faults occur. As shown in FIG. 1B, the crystal defect 23 may cross the junction between the N.sup.- -type layer 16 and the Si substrate 11.
As a result, junction leakage occurs at a portion where the crystal defect 23 crosses the junction between the N.sup.- -type layer 16 and the Si substrate 11, poor data retention characteristics are obtained in the conventional stacked capacitor DRAM.
As described above, after the SiO.sub.2 film 12 is formed by the LOCOS method, when the entire surface of the SiO.sub.2 film 12 is etched without using a mask, the position of the bird's beak 12a which is the thin portion of the SiO.sub.2 film 12 is recessed to release the latent stress 22, and the occurrence of the crystal defects 23 is suppressed. In this case, however, the thickness of the SiO.sub.2 film 12 except for the bird's beak 12a is also decreased, thereby degrading isolation characteristics.